In many neurodegenerative diseases such as Alzheimer's disease (AD), tau protein aggregates accumulate in brain cells, generally referred to as “tauopathies.” Of these, in familial frontotemporal lobar degeneration (FTLD) (known as frontotemporal dementia and Parkinsonism linked to chromosome 17 (FTDP-17)), genetic mutations in tau genes have been discovered. After that, a study of Tg mice that overexpressed human wild type (WT) or FTDP-17 mutant tau proteins has made it clear that tau amyloid production takes part in the mechanism of neurodegenerative episodes in Alzheimer's disease (AD) and non-Alzheimer-type (non-AD) tauopathies (non-patent literature 1). Also, it has been shown that tau protein aggregates in AD, referred to as neurofibrillary tangles (NFT), are closely linked to disease severity than senile plaques that are made of amyloid β peptides (Aβ) (non-patent literature 2). By contrast with amyloid precursor protein (APP) Tg mice in which Aβ aggregates accumulate without a decrease of neurons, tau Tg mice exhibit a significant decrease of neurons (non-patent literature 3). It is therefore necessary, in future studies, to make the neurotoxicity of fibrous tau proteins in tauopathies pathologically clear, by a comparative evaluation of the living human brain and the mouse brain.
In vivo imaging—for example, positron emission tomography (PET), optical imaging, and nuclear magnetic resonance imaging—is able to visualize Aβ deposits in AD patients and AD mouse models in vivo. As molecular probes to be used thereupon, compounds such as [18F]FDDNP, [11C]6-OH-BTA-1(PIB), [11C]AZD2184, [11C]BF-227, [18F]-BAY94-9172, and [18F]AV-45 are known (patent literatures 1 to 4). Among these, [18F]FDDNP has been suggested to bind to both senile plaques and NFTs. However, since this compound has binding to the dense core of Aβ aggregates, interactions with tau pathologies in AD patients have not been shown clearly. In addition, there is a problem that this compound does not bind to tau aggregates in non-AD tauopathy brains without senile plaques, and therefore cannot directly show binding to tau pathologies in vivo. Consequently, development of novel compounds that specifically bind to tau proteins that accumulate in the brain due to AD and non-AD tauopathies, and that allow imaging of tau aggregates, has been sought after.